Paper-based containers with barrier properties are typically formed from a paper-based blank comprising a paperboard substrate and functionalized layers, such as oxygen and moisture barrier layers. The blank is die cut to the desired silhouette and then formed into a shape by wrapping it once around a mandrel. The overlapping ends of the blank form a straight seam having an underlying portion and an overlying portion. FIG.1 shows a cross sectional view of the container body 100 made by overlapping ends of the blank 101 into a straight seam having an underlying portion 102 and an overlying portion 103. The raw edge 104 of the underlying portion of the seam is exposed to the container content, resulting in a reduction of the barrier performance of the container. Several techniques have been reported to prevent the raw edge 104 of the seam from being exposed to the packaged content.
One approach is to cover the exposed raw edge of the blank with a strip of barrier tape. Examples of materials used as a barrier tape to protect the raw edges of paperboard containers include metal foils, such as aluminum foil and tin foil, low density polyethylene (LDPE), ethylene-vinyl alcohol copolymer (EVOH), polyethylene terephthalate (PET), glycol modified PET, nylon, and combinations thereof. U.S. Pat. No. 5,620,135 discloses a technique for covering the raw edge of the body with a protective covering tape. The container body is formed from a blank having one longitudinal edge enclosed by a U-shaped protective covering. Prior to formation of the container body, the covered region of the blank is compressed to a reduced thickness such that the entire sheet segment, including the protective covering, has approximately the same thickness. PCT Application No. WO 2003/106277 discloses a single wrap container having the exposed underlying edge of the paper-based container body enclosed by a tape that comprises a layer of metalized PET interposed between layers of LDPE. The container body is formed from a blank comprising layers of, from the inside out, polyolefin, paper stock, polyolefin, metal foil, polyolefin, printed paper and overprint varnish. Using protective tapes to cover the raw edges, however, has several drawbacks. The adhered protective tape is an additional cost, and may be easily removed. Furthermore, an additional process is required to apply the protective tapes, resulting in further increase in cost and complexity of the manufacturing process.
Another approach commonly used in the multi-ply tubular container process is to fold the underlying edge portion of the barrier liner ply into an “anaconda” fold, wherein the underlying edge is folded back on itself and adhere to the overlying edge. An example of such a fold is illustrated in U.S. Pat. No. 5,084,284. The main drawback of an anaconda fold is the undesired increase in thickness of the seam, as it is three times the thickness of the blank. Cracks tend to form with such high thickness, resulting in a leakage of the contents, an influx of the outside air, and a reduction in barrier performance of the containers. Furthermore, such undesirably high thickness of the seam poses difficulties when attempting to hermetically seal the ends of the container body itself, as well as seal the top lid and bottom to the container body. To address the difficulty in folding the paper-based blank during the formation of the container body, several techniques have been used to reduce the thickness of the blank.
U.S. Pat. No. 6,190,485 teaches a technique of manufacturing a hermetically sealed spiral-wound multi-ply container without using an “anaconda” fold. The hollow body component of the hermetically-sealed container comprises a paperboard body ply and a liner ply on the inner surface of the body ply, wherein the inner ply includes a barrier layer and a layer of adhesive that is activatable at a predetermined activated temperature. A continuous body ply formed of paperboard having first and second side edges is advancing towards a shaping mandrel, while a continuous polymeric liner ply having first and second side edges is advancing adjacent to one surface of the paperboard body ply. The liner ply and the body ply are passed in face-to-face contact through a pair of nip rollers with the adhesive layer of the liner ply adjacent to the body ply, such that the first marginal edge of the liner ply extends beyond the first marginal edge of the body ply. The adhesive layer is then heated above its activated temperature so that the liner ply adheres directly to the inner surface body ply. The resulting adhered multi-ply is wrapped around the shaping mandrel such that the second marginal edge of the body ply engages the first marginal edge of the body ply, and the second marginal edge of the liner ply engages the first marginal edge of the liner ply. Finally, the first marginal edge of liner ply is heated to a temperature above the activated temperature of the adhesive so the second marginal edge adheres to the first marginal edge, resulting in a tubular shape that is subsequently cut into discrete lengths to provide the hollow body component of the hermetically sealed paperboard container. This process of producing hermetically sealed containers uses continuous webs of paperboard ply and liner ply, which require rather intensive handling, as well as relatively high shipping and storage costs.
Another approach of protecting the raw edge of the blank is “skiving and hemming,” as described in U.S. Pat. No. 5,236,408. Skiving is removing some amount (e.g., half) of the thickness of the paperboard from the side seam flap. Hemming is folding the skived area back onto itself and sealing the other edge over the hemmed area by heat or flame. This approach has been used for producing gable top cartons for the liquid packaging industry, as described in U.S. Pat. No. 5,810,243. While the raw edge of the vertical side seam of the carton is protected, special sealing techniques using sealing jaws are required, such as those described in International Patent Application Nos. WO 2008/025996 and WO 1990/009926, to seal the folded top and bottom flaps of a hermetically sealed carton.
Achieving a hermetically sealed barrier paperboard cup using the “skiving and hemming” approach is difficult on the paperboard cup forming machine, which is typically designed to produce liquid-tight containers with a top rim not designed for a gas tight seal. The increased thickness of the skived/hemmed edge area over the original paperboard provides an additional challenge in producing a hermetically sealed bottom in the area where the thick skived/hemmed seam meets the bottom in the overlapped area. The skived edge also substantially increases the abrupt step at the seam of the top rim formed by the overlapped ends of the blank, and the non-planar hill-like surface of the rim makes hermetically sealing the lidding membrane (film or paper) more difficult.
GB Patent Application No. 2055743 discloses a paper-based container comprising a hollow container body having a recessed structure on the upper and lower ends, a top lip positioned on the recessed top of the container body, and a bottom positioned on the recessed bottom. The hollow container body is produced by skiving one longitudinal end of a paper-based blank to substantially half its thickness for a predetermined width and then forming a longitudinal groove substantially at the center of the skived portion. A heat-resistant adhesive, such as vinyl acetate emulsion and the like, is applied to the skived portion, and the adhesive is irradiated with infrared rays to evaporate water contained therein. The skived portion is then folded about the groove so that the end face of the skived paper and the end face of the unskived portion contact each other. Subsequently, the container body is formed by adhering both longitudinal ends of the blank together in overlapping relation with the folded longitudinal end inside the other longitudinal end. The paper-based blank consists of, from the outside: a thermo plastic synthetic resin layer, a paper layer, an adhesive layer, a metal layer, a heat-resistant adhesive layer, and a thermoplastic synthetic resins layer. The top lid and bottom components may be made of the same or similar material as that for the body component. This process of producing a barrier container is, however, rather complicated and high cost due to the use of adhesives and the recessed structure of the upper and lower ends.
Up until the present disclosure, to the inventor's knowledge, hermetically sealed barrier paperboard cups have not been achieved commercially using the “skiving/hemming/flame sealing” approach without adhesives on regular cup forming machines.
Accordingly, there is still a need for hermetically sealed paperboard containers with enhanced barrier and seal performances that may be produced by a process that is more effective and economical using commercially available high-speed liquid packaging skiving/hemming/sealing equipment and cup forming machines without the use of adhesives. One advantage of such approach is the potential for the in-plant system, where skived blanks can be shipped flat to the packaging plant where the barrier cups are formed using in-plant cup forming machines.
It is further beneficial to have hermetically sealed paperboard containers with excellent barrier performance that do not require the use of metal foils to impart the barrier properties.